EAGER:SUPER: In-situ Synthesis of a New Functional Material: Superconducting Polyacetylene

想象一个拥有无限可充电电池的世界，这些电池非常轻，储存电能而不会损失，电线不会失去电力，便携式医疗成像扫描仪——这些都是一些变革性的应用，可以通过室温（RT），环境压力超导体（无电阻传输电力的材料）的发展来实现。理论预测线性分子导体，如聚乙炔（PA），其碳原子链通过交替的双键和单键连接，可能就是这样一种超导材料。在实验室中，PA合成时绝不是线性的，它变得高度无序（像意大利面一样），容易与自身或邻近分子发生反应。就像一条蜿蜒的道路会减慢交通速度一样，一条笔直的PA链可以扭曲成弯曲和扭曲的几何形状，从而显著影响其导电性——这一因素本身就是在这种和其他潜在的有机导电聚合物中实现RT超导性的主要实验瓶颈。在该项目的支持下，锡拉丘兹大学材料研究部的Michael Sponsler教授和他的研究小组将专注于制备一种全新形式的PA，该PA具有分离和高度有序的单链，隔离在长而直的隧道中。为了获得能够表现出潜在RT超导性的线性几何结构，PA链将在蜂窝状尿素晶体的隧道中合成，然后研究确认其几何形状和电学行为。如果成功，该项目将首次生产出结构有序的PA，产生一种新的有机材料，这将对导电聚合物的理论和应用产生重大影响，并揭示RT，环境压力超导状态，这是一个长期难以实现的目标。此外，整合大量的实验成果来表征这些链及其行为，将有助于培养从本科到博士后水平的新化学家，包括女性和这些跨学科研究项目中代表性不足的少数族裔。聚乙炔（PA）是最简单的共轭聚合物，被认为是一种具有键长交替（BLA）的长直链半导体。基于预测PA链不可能有BLA的基础计算，因为振动零点能量高于Peierls势垒，本研究的前提是，完全扩展的全反式构象中的单个孤立PA链将具有半填充的能带电子结构，因此将是一维（1D）未掺杂的金属聚合物。此外，与一维电导率相关的碳-碳（C-C）键交替模式的振动频率（1455 1/cm）比室温（RT）下的热能高约7倍，排除了热激发作为电子散射机制的可能性。由于不存在抵抗电子运动的机制，因此PA的高温超导性是一种现实的可能性。该项目由材料研究部支持，将测试一个假设，即足够长度和隔离的单个PA链将通过以下方式显示超导性：(1)通过将聚合物链包裹成纳米直径的结构（尿素包裹晶体的平行隧道）来合成有序的、完全延伸的PA链；(2)表征所产生的长链的结构和电子特性，以证明它们没有BLA，并且对直流电流没有可测量的电阻，表明PA作为RT，环境压力超导体。PA/尿素包合晶体将由尿素和光反应前体分子（E，E，E）-1,6-二碘己三烯（DIHT）通过一种称为消除-缩合包合聚合的新型光诱导工艺制成。这种独特的方法利用DIHT分子末端碳-碘（C-I）键的光化学键断裂，形成新的C-C键，并消除尿素宿主晶体隧道中的碘。光化学过程继续进行，直到所有的碘都离开晶体，高分子量和完全共轭的PA链在结晶尿素通道内形成。所包括的DIHT到PA的光化学转化将通过质量损失测量来监测，以跟踪从初始晶体中损失的碘的化学计量量，并通过拉曼光谱来监测在该过程的初始阶段观察到的特征的生长和消失。所得的PA/尿素晶体将通过高分辨率x射线衍射、拉曼光谱、非弹性中子散射光谱和电导率测量进行研究，包括块状晶体测量和使用导电原子力显微镜。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Non-technical SummaryImagine a world with infinitely rechargeable batteries that are very light, store electrical energy without losses, power lines that lose no power, and portable medical imaging scanners — these are a few of the transformative applications that could be realized by the development of room-temperature (RT), ambient pressure superconductors (materials that transport electricity without resistance). Theory predicts that linear molecular conductors, like polyacetylene (PA), whose chains of carbon atoms are connected by alternating double and single bonds, might be one such superconducting material. In the laboratory, PA is anything but linear when synthesized, becoming highly disordered (like a spaghetti) and prone to reacting with itself or neighboring molecules. Just as a winding road slows traffic, what starts as a straight PA chain can twist to produce bent and twisted geometries that significantly affect its conductivity — this factor alone is a major experimental bottleneck in achieving RT superconductivity in this and other potential organic conducting polymers. With this project, supported by Division of Materials Research, Professor Michael Sponsler and his research group at Syracuse University will focus on preparing a completely new form of PA with separated and highly ordered single chains isolated in long, straight tunnels. To achieve a linear geometry capable of exhibiting potential RT superconductivity, the PA chains will be synthesized within the tunnels of honeycomb-like urea crystals, then studied to confirm their geometries and electrical behavior. If successful, the project will produce structurally ordered PA for the first time, result in a novel organic material that will have a significant impact on the theory and application of conducting polymers, and reveal the RT, ambient pressure superconductivity state, a long-standing elusive goal. Moreover, integrating a large arsenal of experimental efforts to characterize these chains and their behavior will serve to educate new chemists from undergraduate to post-doctoral levels, including women and underrepresented minorities in these interdisciplinary research projects.Technical SummaryPolyacetylene (PA), the simplest conjugated polymer, is thought of as a long, straight-chain semiconductor exhibiting bond-length alternation (BLA). Based on foundational calculations predicting that PA chains cannot have BLA because the vibrational zero-point energy is above the Peierls barrier, the premise of the proposed study is that an individual, isolated chain of PA in its fully extended all-trans conformation will have a half-filled band electronic structure and thus will be a one-dimensional (1D) metallic polymer without doping. Further, the vibrational frequency of the carbon-carbon (C-C) bond alternation mode (1455 1/cm) associated with the 1D conductivity is ~7 times higher than thermal energy at room temperature (RT), precluding thermal excitation as a mechanism of electron scattering. Since there is no mechanism for resistance to the electron motion, high-temperature superconductivity in PA is a real possibility. This project, supported by the Division of Materials Research, will test the hypothesis that individual PA chains of sufficient length and isolation will show superconductivity by (1) synthesizing ordered, fully extended PA chains by encasing the polymer chains into structures with nanometer-sized diameters – the parallel tunnels of a urea inclusion crystal; and (2) characterizing the structural and electronic properties of the long chains produced to demonstrate that they show no BLA and have no measurable electrical resistance to DC currents, showing that PA acts as an RT, ambient pressure superconductor. The PA/urea inclusion crystals will be made from urea and a photoreactive precursor molecule, (E,E,E)-1,6-diiodohexatriene (DIHT), through a new light-induced process called elimination-condensation inclusion polymerization. This unique approach employs photochemical bond scission of terminal carbon-iodine (C-I) bonds from a DIHT molecule with the formation of new C-C bonds and elimination of iodine from the tunnels of the urea host crystal. The photochemical process continues until all iodine has left the crystal and high molecular weight and fully-conjugated PA chains are made within the crystalline urea tunnels. The photochemical conversion of included DIHT to PA will be monitored by mass-loss measurements to track the stoichiometric amounts of iodine lost from the initial crystal and by Raman spectroscopy that monitors the growth and disappearance of features observed in the initial stages of the process. The resulting PA/urea crystals will be studied by high-resolution X-ray diffraction, Raman spectroscopy, inelastic neutron scattering spectroscopy, and conductivity measurements, both in bulk crystal measurements and by the use of conductive atomic force microscopy.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.